Linear gradient with non-linear easing

main.dart

import 'package:flutter/material.dart';
import 'dart:ui' as ui;

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            CustomPaint(
              painter: FooPainter(Curves.linear),
              size: Size(200, 200),
            ),
            SizedBox(width: 16),
            CustomPaint(
              painter: FooPainter(Curves.easeIn),
              size: Size(200, 200),
            ),
          ],
        ),
      ),
    );
  }
}

class FooPainter extends CustomPainter {
  FooPainter(this.curve);

  final Curve curve;

  void paint(Canvas canvas, Size size) {
    var paint = Paint();
    paint.shader = ui.Gradient.linear(
      Offset(size.width / 2, 0),
      Offset(size.width / 2, size.height),
      [Colors.yellow, Colors.blue],
      [0, 1],
    );
    var startColor = Colors.yellow;
    var endColor = Colors.blue;

    var colors = <Color>[];
    var stops = <double>[];
    for (var i = 0; i < 32; i++) {
      var value = (i / 32.0).clamp(0.0, 1.0);
      var transformed = curve.transform(value);
      colors.add(Color.lerp(startColor, endColor, transformed)!);
      stops.add(value);
    }

    paint.shader = ui.Gradient.linear(
      Offset(size.width / 2, 0),
      Offset(size.width / 2, size.height),
      colors,
      stops,
    );
    canvas.drawRRect(
        RRect.fromRectAndRadius(Offset.zero & size, Radius.circular(8)), paint);
  }

  bool shouldRepaint(covariant FooPainter painter) {
    return true;
  }
}